When the true sap velocity is negative or very low, the uncertainty inV 23 will be greater than that forV 12 and V DRM will equalV 12, whereas at high velocities,σ 12 >σ 23 andV DRM will equal V 23. This
selection procedure avoids the need to specify an arbitrary value of sap
velocity at which to switch from one estimate to the other, as is
required in the approach of Pearsall et al. (2014).
2.1.4 Identifying the optimal time window for averaging sap velocity
estimates
In the HRM, V 12 is computed as the average of
repeated instantaneous estimates during a fixed period after the heat
pulse. ICT International suggests a 40 s time window, beginning 60 s
after the pulse and ending at 100 s. The uncertainty-based approach of
the DRM provides an objective way to identify the optimal time window:
namely, by choosing the period that minimizes intrinsic uncertainty in
sap velocity, computed as the standard error of the mean estimated
velocity (based on the theoretical uncertainty inV DRM, not the standard deviation of a sample of
instantaneous estimates):